1. Field of the Invention
The present invention relates to a process for forming a thin film of amorphous silicon on the surface of an insulating substrate such as a glass plate.
2. Description of Related Art
Thin films of amorphous silicon have been fabricated heretofore by a plasma enhanced chemical vapor deposition process (hereinafter abbreviated to "PECVD") comprising:
introducing a silane (SiH.sub.4) gas or a mixture of silane gas and hydrogen (H.sub.2) gas as a starting material into a film deposition (vacuum) chamber; PA1 applying a high frequency power across a pair of facing electrodes to produce a plasma by electric discharge; and PA1 exciting and decomposing the starting material gas to form a thin film of amorphous silicon on the surface of a substrate supported by one of the electrodes.
This film deposition process has been applied to the production of thin film transistors (TFTs) for use in liquid crystal displays (LCDs) or in flat panel displays. In general, the TFT elements are formed on a glass plate. However, the size of the glass plate has been getting larger recently because the size of the liquid crystal displays has been increasing. A key point in the fabrication of large liquid crystal displays of high quality is to deposit a film having a uniform film thickness. Thin films of amorphous silicon are no exception, and uniformity in the film thickness is recognized as an important factor to be fulfilled in their deposition.
In forming a thin film of amorphous silicon by PECVD, it is well known that the film thickness uniformity depends on the distribution of the plasma density at the surface of the substrate. That is, it is known that the PECVD reaction can be promoted by increasing the plasma density, and therefore films can be deposited at a higher rate by increasing the plasma density. This also means that portions of a film deposited where the plasma density is high will be thicker than portions of the film deposited where the plasma density is lower. Therefore, to obtain a good film with a uniform thickness, a plasma with a uniform density distribution must be formed between the facing electrodes.
The discharge between the facing electrodes can be effected in either of two ways: one is a continuous discharge method; the other is an intermittent discharge method in which a square wave amplitude-modulated discharge is used.
The continuous discharge method is the PECVD process that has generally been employed, and is characterized in that it enables the deposition of high-quality thin films of amorphous silicon while maintaining the substrate at a relatively low temperature of about 250.degree. C. When depositing a thin film of amorphous silicon on a glass plate having a large area (e.g., 20.times.20 cm.sup.2), however, this method is problematic because such a large thin film of uniform thickness cannot be obtained.
The method using intermittent discharge comprises applying a square wave amplitude-modulated radio frequency (rf) between the facing electrodes. The other basic film deposition conditions (pressure, substrate temperature, the composition and the flow rate of the starting material gas) are the same as those used in the continuous discharge method. The square wave amplitude-modulated discharge method was initially proposed by Overzet et al. (see L. J. Overzet et al., Appl. Phys. Lett. 48(11), pp. 695-97 (1986)). This method has been studied in further detail by Watanabe et al. of Kyushu University. See Y. Watanabe et al., Appl. Phys. Lett. 53 (14), pp. 1263-65 (1988), M. Shiratani et al., IEE Japan, Proceedings of Symposium on Plasma (Plasma Kenkyu-kai Shiryou), EP-89-62 (1989), and Y. Watanabe et al., Appl. Phys. Lett. 57(16), pp 1616-18 (1990).
Watanabe et al. have reported that a film of high quality can be formed by applying the intermittent discharge method, i.e., the method of square wave modulating an rf discharge, to a conventional PECVD process without preventing the formation of radicals necessary for forming a film of high quality and while greatly suppressing the generation of dust particles throughout the entire discharge space. They have reported conclusively that their process enables the formation of a thin film at a high deposition rate while considerably reducing the particle diameter and quantity of the dust particles which form in the discharge space.
Furthermore, Denisse et al. of Utrecht State University have reported the application of the intermittent discharge method to a process for depositing a thin film of SiO.sub.x N.sub.y. See C. M. M. Denisse et al., J. Appl. Phys. 60(7), pp. 2536-42 (1986).
A conventional process for depositing a film of SiO.sub.x N.sub.y comprises carrying out ordinary continuous discharge PECVD by introducing a mixture of SiH.sub.4, N.sub.2 O, and NH.sub.3 gases into the vacuum vessel. However, the composition ratio of the gas mixture during this process changes from the vicinity of the gas inlet to the vicinity of the gas outlet, resulting in a film whose thickness differs from place to place. This problem of the composition ratio changing occurs because the plasma reaction consumes the gas mixture introduced through the inlet as the gas flows through the vacuum vessel.
As a solution to this problem, Denisse et al. proposed the use of an intermittent discharge method in depositing thin films. They used a 1 kW radio frequency generator operating at 400 kHz and being modulated by a pulse with a frequency of about 4 Hz (about 250 ms per cycle) and a pulse width of about 100 ms. The plasma was generated intermittently with discharge occurring for a duration of about 100 ms and ceasing thereafter for about 150 ms to allow the recovery of the initial composition ratio. Thus, the gas composition ratio in the discharge space could be maintained constant and film of uniform thickness could be deposited.
However, the PECVD processes using the intermittent discharge method described above do not enable the deposition of a thin film of amorphous silicon with uniform thickness over the entire surface of a large insulating substrate (e.g., a glass plate 20.times.20 cm.sup.2 in area).
As described in the foregoing, a PECVD based on the continuous discharge method is disadvantageous in that it cannot be applied to the deposition of a thin film of amorphous silicon having a uniform thickness on a large glass plate. FIG. 3 shows an example of a film deposited by the continuous discharge method. It can be seen that the film is relatively thin in the central portion of the substrate and that it is thicker at the edges of the substrate, i.e., at portions near the metallic substrate holder which is grounded.